Multilayer heat shrinkable films have been known for a long time.
U.S. Pat. No. 4,532,189 (W.R. Grace & Co.) discloses a multilayer, heat shrinkable film comprising:
(A) a core (central) layer comprising a linear low density polyethylene or a linear medium density polyethylene; PA1 (B) two skin (external) layers comprising a blend of from 70% to 90%, by weight, of an ethylene/propylene copolymer and from 10% to 30%, by weight, of a propylene homopolymer; PA1 (A) a cross-linked core layer consisting essentially of a linear low density polyethylene; and PA1 (B) two surface layers comprising essentially a blend of (1) a linear low density polyethylene, (2) a linear medium density polyethylene and (3) an ethylene/vinyl acetate copolymer. PA1 (A) 5-100%, by weight, of at least one linear copolymer of ethylene with at least one C.sub.8 -C.sub.18 alpha-olefin, said copolymer having the following characteristics: PA1 (B) 0-95%, by weight, of at least one polymer selected from the group consisting of ethylene homopolymers and copolymers of ethylene with an ethylenically unsaturated comonomers, said polymer having only one crystallite melting point below 128.degree. C.; PA1 (A) 75-100%, by weight, of at least one linear ethylene/alpha-olefin copolymer having a density of between 0.890 g/cm.sup.3 and 0.930 g/cm.sup.3, and PA1 (B) 25-0%, by weight, of a linear high density polyethylene having a density of between 0.935 and 0.960 g/cm.sup.3, with the proviso that the total of the film-forming polymer (A) and (B) has a single melting point as determined by differential scanning calorimetry according to ASTM D-3417. PA1 This property is measured by gradually heating the film and measuring the maximum force developed per section unit of the film itself. PA1 This property is measured by heating the film at 120.degree. C. for at least 5 sec., and then measuring the percentage of shrinkage in the longitudinal direction and in the cross-direction. PA1 This property is measured according to ASTM D 1003-61. PA1 This property is measured according to ASTM D 2457-90. PA1 This property is measured according to ASTM D 882-90. PA1 This property is measured according to ASTM D 882-90. PA1 This property is measured according to ASTM D 882-90. PA1 This property is measured according to ASTM D 374-88. PA1 The main planarity defects consist of bag & sag and snaking. PA1 The device used to measure these defects essentially consists of PA1 In order to perform the measurements, a roll of film is placed in the unwind section and a length of film is unwound. PA1 The unwind station is locked. PA1 The film is aligned and fixed to the wind-up core. PA1 The rewind section is locked. PA1 Using the vertical scale, the position of the flattest part of the film is measured. PA1 The bag and sag in the film are then measured by recording the difference between the film levels at each point and the level of the flattest part of the film. PA1 Measurements are taken along the whole film width from the left to the right side so as to obtain a representative profile of the surface. For this purpose measurements every 5-10 cm are recommended. PA1 The data are reported on paper, i.e. sag and relevant position (left, center or right). PA1 The whole operation is performed twice to four times to measure from two to four lengths of film. PA1 Snaking, also referred to as "banana effect", represents the deviation of film from a straight line for a given length of material. PA1 The measurements are performed on a length of film of 4 meters using the device and method described above. PA1 Cross-linking is measured according to ASTM D 2765-90. PA1 A.sub.1) 50-100 parts, by weight, of an ethylene/alpha-olefin C.sub.4 -C.sub.12 copolymer or a blend of ethylene/alpha-olefin C.sub.4 -C.sub.12 copolymers, and PA1 A.sub.2) 50-0 parts, by weight, of a polymer selected from polyolefins, modified polyolefins and blends thereof, PA1 A.sub.1) 50-100 parts by weight of an ethylene/alpha-olefin C.sub.4 -C.sub.12 copolymer or a blend of ethylene/alpha-olefin C.sub.4 -C.sub.12 copolymers, and PA1 A.sub.2) 50-0 parts by weight of a polymer chosen from among polyolefins, modified polyolefins and blends thereof,
wherein said film has an average machine direction free shrink at 200.degree. F. of at least 12% and an average cross-direction free shrink at 200.degree. F. of at least 17%.
The core layer may also comprise other polymers such as, for example, ethylene/propylene copolymers, ethylene/vinyl acetate copolymers, ionomer resins and non-linear low density poly ethylenes.
Moreover, said film may also comprise two intermediate layers comprising a blend of approximately 90% by weight of an ethylene/vinyl acetate copolymer and approximately 10% of an ionomer resin.
U.S. Pat. No. 4,551,380 (W.R. Grace & Co.) discloses a multilayer heat shrinkable film comprising:
GB-A-2,097,324 discloses heat shrinkable films manufactured by stretching, at least three times their original linear dimension in at least one direction, a film having the following homogeneous composition:
(a) melt index of 0.1-4.0 g/10 min.; PA2 (b) density of 0.900-0.940 g/cm.sup.3 ; PA2 (c) stress exponent above 1.3; and PA2 (d) two distinct crystallite melting regions below 128.degree. C. as determined by differential scanning calorimetry (DSC), the temperature difference between said regions being at least 15.degree. C.; and PA2 1. an unwind section with locking devices, PA2 2. a dancing rolls to allow proper tension adjustment, PA2 3. two idle rolls spaced circa 4 meters one from the other to support the film span under measurement, PA2 4. a reference scale for proper film alignment close to each idle roll, PA2 5. a pair of rubber clamps actuated by a pedal lever to block the film side opposite to the unwind section, PA2 6. a motorised rewind section, PA2 7. a device for measuring the length of the rewound film between two sets of measurement, PA2 8. a guide, positioned substantially at the center between the two idle rolls, which supports: PA2 9. various couples of weights to be attached to the dancing roller to adjust the tension of the film according to the type, width and thickness of the film itself.
with the proviso that stretching is carried out within the temperature range defined by the two melting points of the crystallites of the ethylene/alpha-olefin copolymer of the above paragraph (A).
These films are manufactured by the well-known air bubble technique. An example of method and equipment of this technique is disclosed by U.S. Pat. No. 4,841,605.
However, the films obtained with this technique have the disadvantage of not having sufficiently uniform thickness and planarity. In fact the total thickness variation in said films is .+-.15% while, as regards planarity, it has defects consisting of deviations from a straight line (snaking) and sags. More particularly the average deviations from a straight line (snaking) is approximately 50 mm, whereas the average sag is approximately 35 mm.
In addition to the air bubble technique, stretching the films also by the so-called "tenter frame" technique is known. Examples of machines suitable for implementing this technique are disclosed by U.S. Pat. Nos. 3,148,409 and 3,201,826.
The tenter frame technique, also known as "flat orientation technology", consists of extruding a film-forming material through a flat die over a chill roll, preferably immersed in water, to chill the molten film.
In the case of multilayer films the various polymers or blends of polymers are generally coextruded by conventional techniques but, when only a few of the layers have to undergo special treatments, such as for example irradiation with fast electrons to induce cross-linking, only the layer or layers to be treated is extruded or are coextruded, the tape obtained in this way is subjected to the required treatment and then the remaining layers are extruded on the same.
The tape is then oriented, by stretching, in two separate and successive steps, although devices able to stretch the tape simultaneously in both directions are known (U.S. Pat. No. 3,148,409).
Generally stretching is performed first in machine direction (MD) and then in cross-direction (TD).
MD stretching is usually carried out by passing the tape through pairs of rolls which rotate at different speeds. At least one of the first pairs of rolls is heated, for example by inner circulation of hot oil.
TD stretching is usually performed in a tenter frame oven which comprises a certain number of heating zones and suitable stretching means.
Typically a tenter frame oven comprises from three to six zones: one to two for preheating the tape, one to two for stretching it in the cross-direction and one to two for relaxing and winding the film. Each zone may be heated at a different temperature level.
This technique has not however been adopted in the manufacture of heat shrinkable films based on ethylene copolymers.
Only EP-A-405 916 discloses the utilization of this technique in the manufacture of an extruded, bioriented, mono or multilayer film, wherein the film-forming polymers of at least one layer consist of:
Moreover, EP-A-405 916, page 3, lines 18-21, reports that attempts made to apply the tenter frame technique to the polymers of GB-A-2 097 324 have not given satisfactory results because the films obtained in this way were highly sensitive to minimal variations of the process parameters, such as stretching temperature, stretching ratio and the speed of the manufacturing line.
Although EP-A-405 916 also refers to multilayer films, its examples only refer to monolayer films.
On the other hand, multilayer films have, compared to monolayer ones, the considerable advantage of allowing to combine one with the other several layers having different physical and chemical properties in view of the required properties of the final film.
Therefore the need for heat shrinkable multilayer films, extruded from a flat die, bioriented by the tenter frame technique, wherein at least one layer comprises ethylene copolymers, is still greatly felt.
In fact, the films manufactured by this technique have several advantages over those manufactured by the air bubble technique.
A first advantage consists of the fact that the stretching ratios in machine direction and cross-direction may vary as required whereas in the air bubble technique they are always substantially equal one to the other.
A second advantage is that the stretching ratio may be preselected within a relatively wide range, typically of betweeen 2:1 and 12:1, while in the air bubble technique it must be between 3:1 and 6:1.
A third advantage is that the sealing agents can be selected as required whereas in the air bubble technique they have to be selected in the restricted range of sealants whose softening point is not substantially lower than the stretching temperature to prevent the sealant from softening during the heating step, with consequent sealing of the opposite walls of the bubble.
A further advantage consists of the fact that the thickness and planarity of the films manufactured by this technique are much more uniform than in those manufactured with the air bubble technique.
An additional advantage is that, being able to control the thickness better, thinner films can be manufactured, consequently saving materials and reducing environmental impact.